Patent GB-701830-A (GB)
Title: Television apparatus
IPC: H04N11/12
Filed: 1951-09-14
Abstract: 701,830. Colour television. HAZELTINE CORPORATION. Sept. 14, 1951 [Oct. 14, 1950], No. 21657/51. Class 40 (3) Relates to television system utilizing shared-band multiplex transmission and is described as applied to the following types of systems:- (1) A sequential sampling monochrome system, Figs. 1 and 2 (not shown) as described in Electronics Dec. 1949 in which a 0-8 Mc/s video signal is sampled at a recurrence frequency of 8 Mc/s and applied to the transmitter through a 0-4 Mc/s lowpass filter so that the 0-4 Mc/s low-frequency components are reconstituted in the filter and the 4-8 Mc/s high-frequency components are translated through the filter as the 0-4 Mc/s lower position of the lower sidebands of the 8 Mc/s sampling frequency. At the receiver the detected video signals are applied to an 8 Mc/s sampler operating synchronously with the transmitter sampler which passes the 0-4 Mc/s low-frequency component signal and the 0-4 Mc/s high-frequency component sideband signal and also heterodynes them with the 8 Mc/s sampling frequency to give a 4-8 Mc/s lowfrequency component sideband signal and a 4-8 Mc/s high-frequency component signal. If the sampling frequency Fs is an odd harmonic of half the line frequency F1 then the two sets of sideband signals tend to cancel one another on alternate fields but their presence results in the reproduced image having a dotted structure. In such a system the sampling signal may be considered to be a subcarrier which is suppressed at the transmitter by the 0-4 Mc/s filter and reinserted at the receiver by the receiver sampler and accordingly the transmitter and receiver samplers may be replaced by a modulator and demodulator supplied with a c.w. 8 Mc/s subcarrier. (2) A " dot sequential " colour system, Fig. 3 and Fig. 4 (not shown) of the type described in R.C.A. Review, Dec. 1949, and the modification thereof described in Specifications 689,356 and 689,821 in which three component colour signals are sequentially sampled at the transmitter at a recurrence frequency of 3-8 Mc/s, the resultant signal being synchronously re-sampled at the receiver. (3) A frequency-interlace colour system, Fig. 11 (not shown) as described in Electronics, Sept. 1950, in which the green signal and the highfrequency components of the red and blue signals are transmitted normally through a 0-4 Mc/s filter, the low-frequency components of the red and blue signals being modulated on to respective subcarriers in the 0-4 Mc/s band which are different odd harmonics of half the line frequency. In all the above systems the reproduced image has a dotted structure and for a type (1) system employing conventional line interlace in which FS=(2n+1)/2 FL the dot structure is as shown in Fig. in which the numbers 1-4 indicate the dots laid down in successive fields 1-4. In this pattern dots occurring in consecutive fields are arranged in vertical consecutive order resulting in a vertical " crawl " of the image and according to the present invention this effect is reduced by choosing the sampling frequency such that the dots on adjacent lines in space of the completed image are relatively displaced resulting in a low visibility dot structure. The use of cascade sampling is also described whereby transmission through a bandwidth less than 0-4 Mc/s may be effected. Examples of dot patterns produced in accordance with the present invention are shown in Figs. 7-10. For simplicity of illustration the patterns are those produced of a type (1) monochrome system but similar patterns would be produced by a dot sequential colour system if each dot in the figures is replaced by a spaced sequence of colour dots, e.g. red, blue and green. Fig. 7 may be regarded as derived from Fig. 6 by omitting the third and fourth fields and may be produced by making the sampling frequency PS equal to an integral multiple of the frequency F1 plus an odd multiple of half the field frequency Ff, i.e. Fs=nFL+(2N+1)/2 FL, or by making the sampling frequency equal to a multiple of the line frequency and shifting the phase of the sampling frequency by 180 degrees during alternate fields. Fig. 8 is the pattern produced by a two field system in which FS= (n+1/3) FL, Fig. 9, is produced by a four field system in which FS=(2n+1)/2 FL as in Fig. 6 but in which the phase of the sampling frequency is shifted by 90 degrees during the even fields and Fig. 10 may be regarded as derived from Fig. 8 by filling in the spaces by dots produced during fields three and four and is produced by shifting the sampling frequency used to produce the pattern of Fig. 8 by an odd multiple of one quarter field frequency, i.e. FS= (N+1/3)FL+(2N+1)/2 Ff, or by shifting the phase of the Fig. 8 sampling frequency by 180 degrees during the third and fourth fields. In order to produce the above patterns the frequency of the sampling device at the transmitter and receiver may be controlled by the conventional line and field frequency generators and in the examples in which the phase of the sampling frequency is shifted, the phase shifter may be controlled by a multivibrator controlled by the field frequency generator. Alternatively the transmitter sampling frequency may be crystal controlled and the line and field frequencies derived therefrom by frequency subdivision, the receiver sampling frequency being controlled by separately transmitted timing signals and the line and field frequencies being obtained as subharmonics of the timing signal. Dot sequential colour system.-At the transmitter, Fig. 4 (not shown), the 0-2 Mc/s components of the green, red and blue signals are applied to a 3.8 Mc/s three-phase colour sampler and the 0-8 or 2-8 Mc/s components of the colour signals are combined to form a monochrome brightness signal or a "mixed highs" signal respectively which is applied to a 7-6 Mc/s single-phase monochrome sampler whose output is applied to a 0-4 Mc/s filter and combined with the output from the 3-8 Mc/s sampler. At the receiver, Fig. 3, the 2-4 or 0-4 Mc/s components of the video signal selected by filter 52 are sampled by a 3-8 Mc/s three-phase colour sampler 54a and the three outputs are applied through 0-2 Mc/s filters 55a, 55b and 55c and individually combined with the reconstituted brightness monochrome or "mixed highs" signal obtained by applying the input video signal to a 7-6 Mc/s single-phase monochrome sampler 54b and passing the output through a 0-8 or 2-8 Mc/s filter 58. The three-colour signals so obtained are then applied to the green, red and blue control electrodes of the image reproduce 5b. In accordance with the present invention the frequencies of the colour and/or monochrome samplers are such that the dot interference pattern produced by at least one sampler individually is of the form illustrated in Figs. 7-10 and are preferably such that the resultant pattern produced by both samplers functioning simultaneously is also of low visibility. Thus if the colour sampling frequency is an odd multiple of half the line frequency and is shifted in phase by 90 degrees during the even fields the colour interference pattern will be as in Fig. 9 and if the monochrome sampling frequency is the second harmonic of the colour sampling frequency, the monochrome interference pattern will be as in Fig. 7 and the interference produced by interaction between the two patterns will also be of low visibility. Alternatively the colour dot structure may be as in Fig. 10 produced by shifting the phase of the sampling frequency Fs=(n+1/3)FL by 180 degrees during the third and fourth fields in which case the second harmonic sampling frequency produces a pattern as in Fig. 8. The phase of the monochrome sampling should be related to the sampling phase of the predominant component colour, e.g. green, and may be in phase or phase quadrature therewith ; in the latter case cross talk between the high-frequency monochrome components and the colour signals is further reduced if unsymmetrical three-phase colour sampling is employed as in Specification 689,356 with the red and green sampling in phase opposition. Cascade sampling, Fig. 5 (not shown),-When it is desired to couple the composite sampled video signal at the transmitter to the modulator through a long length of transmission line having a bandwidth less than 0.4 Mc/s a further sampler may be inserted at each end of the iine operating at a sampling frequency approximately twice the upper frequency limit of the transmission line, the sampling frequency being one of the types described above with reference to Figs. 7-10 resulting in a low-visibility dot structure but being different from that of the normal samplers already employed. When cascade sampling is applied to the dot-sequential colour system of Figs. 3 and 4, colour cross talk in the transmission line is reduced if the band width of the line is 0-4 Mc/s and the frequency of the cascade sampling is the second harmonic of the colour sampling frequency. Frequency-interlace colour system, Fig. 11 (not shown).-As described the green signal and high frequency components of the red and blue signals are applied to the transmitter through a 0-4 Mc/s filter, the low-frequency components of the blue signal are modulated onto a 3.8 Mc/s carrier and applied to the transmitter through a 3.75-4.0 Mc/s filter and the low-frequency components of the red signal are modulated onto a 3.47 Mc/s carrier and applied to the transmitter through a 2.5-3.6 Mc/s filter. At the receiver the video signal is applied directly to the green control electrode of a three-colour cathode-ray tube and through a 1-4 Mc/s filter to the red control electrode and the red and blue modulated subcarriers are separated by 2.5-3.6 and 3.75-4.0 Mc/s filters respectively and applied to separate detectors, the resultant signals being applied through 0-1.0 and 0-0.25 Mc/s filters respectively to the red and blue control electrod